Selective catalytic reduction via electrolysis of urea

ABSTRACT

A method and apparatus for producing ammonia suitable for use as a reductant in a selective catalytic reduction (SCR), a selective non-catalytic reduction (SNCR), or a flue gas conditioning system is provided. A method for treating combustion exhaust gas with ammonia is provided that includes the electrolytic hydrolysis of urea under mild conditions. The electrolysis apparatus includes an electrolytic cell, which may be operatively coupled to an exhaust gas treatment system to provide an apparatus for reducing nitrogen oxides (NO x ) and/or particulate in exhaust gases.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 37 C.F.R. §1.78(a)(4), this application claims the benefitof and priority to prior filed Provisional Patent Application Ser. No.61/320,447, filed Apr. 2, 2010, which is expressly incorporated hereinby reference.

FIELD OF INVENTION

The present invention relates to methods and devices for treatingexhaust gases.

BACKGROUND

There is concern over the environmental impact of emissions from powerplants and other fossil fuel combustion sources. For example, theexhaust gas of coal-fired power plants contains chemical pollutants suchas nitrogen oxides (“NOx”) and sulfur oxides (“SOx”), as well asparticulates, which are also known as “fly ash”. Selective catalyticreduction (SCR) and selective non-catalytic reduction (SNCR) are meansfor converting nitrogen oxides (NO_(x)) into diatomic nitrogen, N₂, andwater, H₂O. In SCR, a catalyst is used in combination with a gaseousreductant, which is added to a stream of flue or exhaust gas and isabsorbed onto the catalyst. In SCNR, the reductant is injected into theflue gas in a furnace within an appropriate temperature window.Additionally, flue gas conditioning with a gaseous reductant can alsoenhance electrostatic precipitator performance for removing fly ash. InSCR, SNCR, and fly ash removal systems, the reductant is typicallyammonia or urea.

For example, commercial SCR systems are typically found on large utilityboilers, industrial boilers, and municipal solid waste boilers and havebeen shown to reduce NO by 70-95%. More recent applications includediesel engines, such as those found on large ships, diesel locomotives,gas turbines, and even automobiles.

The NO_(x) reduction reaction takes place as the gases pass through thecatalyst chamber. Before entering the catalyst chamber the ammonia, orother reductant, such as urea, is injected and mixed with the gases. Thechemical equations for using either anhydrous or aqueous ammonia for aselective catalytic reduction process are:4NO+4NH₃+O₂→4N₂+6H₂O  (Equation 1)2NO₂+4NH₃+O₂→3N₂+6H₂O  (Equation 2)NO+NO₂+2NH₃→2N₂+3H₂O  (Equation 3)The reaction for urea as a reductant instead of ammonia is:4NO+2(NH₂)₂CO+O₂→4N₂+4H₂O+2CO₂  (Equation 4)

The reaction has an optimal temperature range between 350° C. and 450°C., but can operate from 225° C. to 450° C. with longer residence times.The minimum effective temperature depends on the various fuels, gasconstituents and catalyst geometry.

In SNCR systems, the absence of a catalyst increases the temperature forthe reduction reaction. For example, the temperature window forefficient operation of an SNCR system is typically between 900° C. and1,100° C. depending on the reagent and conditions of the SNCR operation.

Compared to urea, ammonia is more reactive, is more easily disperseduniformly into the flue gas stream and is active over a broadertemperature range, as well as being more efficient. Urea, as such, whilealso an effective reductant, forms unwanted byproducts, such as carbonmonoxide (CO) and nitrous oxide (N₂O), both of which are now undercritical scrutiny by environmental authorities.

Commonly urea is thermally hydrolyzed to form ammonia for exhaust gastreatment applications. The hydrolysis of urea to form ammonia can bebroken down into two distinct reactions. The first reaction is a mildlyexothermic reaction, wherein heat is given off as urea hydrolyzes toform ammonium carbamate. The second reaction, in which the ammoniumcarbamate is converted to ammonia and carbon dioxide, is stronglyendothermic, which overall dominates the thermodynamics of theconversion of urea to ammonia and carbon dioxide, i.e., the overallreaction is endothermic. Therefore, the hydrolysis of urea requires asubstantial amount of heat and quickly stops when the supply of heat iswithdrawn. For example, the liberation of ammonia commences at around110° C. and becomes rapid at around 150° C. to 160° C., with or withoutcatalytic assistance.H₂O+(NH₂)₂CO→(NH₂)CO₂ ⁻NH₄ ⁺+NH₃+heat  (Equation 5)(NH₂)CO₂ ⁻NH₄ ⁺+heat→2NH₃+CO₂  (Equation 6)Excess water promotes the hydrolysis reaction, the overall reaction forwhich is as follows:(x+1)H₂O+(NH₂)₂CO+heat→2NH₃+CO₂+(x)H₂O  (Equation 7)

However, under the reaction conditions necessary to affect usefulthroughput, the water quality is important. For example, in aconventional thermal hydrolysis of urea to ammonia for an SCR system, anaqueous solution of urea is atomized through a spray nozzle into aheated vaporization chamber. As such, the excess water is also vaporizedduring the hydrolysis of urea to ammonia, thereby leaving behind anynon-volatile substances such as minerals. Minerals and othernon-volatile substances can adhere to equipment surfaces, such as spraynozzles and the vaporization chamber walls, and build up over time,which may lead to blockage of the spray nozzle or reduced heat transferefficiency to the vaporization chamber. Thus, the water used in thermalhydrolysis systems needs to be demineralized.

Further, the thermal hydrolysis of urea method is also sensitive to thequality of the urea. For example, formaldehyde present in urea cannegatively affect the performance of an SCR system in a way similar tothat of using demineralized water.

In view of the foregoing, the hydrolysis of urea requires an externalheat source to initiate the reaction, even when coupled with combustionengines, and also is sensitive to the extent of demineralization of thewater and the quality of urea used in the hydrolysis. Therefore, moreefficient methods for generating ammonia for exhaust gas treatmentapplications are needed.

SUMMARY OF THE INVENTION

The present invention is premised on the realization that ammonia can beproduced from the electrolysis of urea to supply exhaust gas treatmentapplications, such as selective catalytic reduction (SCR) systems,selective non-catalytic reduction (SNCR) systems, and/or flue gasconditioning systems.

According to one embodiment of the present invention, a method forsupplying NH₃ to an exhaust gas treatment system is provided. The methodincludes producing ammonia by an electrolytic hydrolysis of ureaeffected by applying a voltage difference to an electrolytic cell,recovering at least a portion of the NH₃, and transferring the at leasta portion of the NH₃ to the exhaust gas treatment system. Theelectrolytic cell includes a cathode having a first conductingcomponent, an anode having a second conducting component, urea, and analkaline electrolyte composition in electrical communication with theanode and the cathode, where the alkaline electrolyte composition has ahydroxide concentration of at least 0.01 M or a pH of at least 8. Thevoltage difference is applied across the cathode and the anode, and thevoltage difference is sufficient to effect the electrolytic hydrolysisof urea to produce at least NH₃.

According to another embodiment of the present invention, there isprovided a method for reducing nitrogen oxide (NO_(x)) emissions and/orparticulate matter in a combustion exhaust gas during ongoing operation.The method comprises injecting ammonia into the combustion exhaust gasupstream of at least one of a selective catalytic reduction (SCR)device, a selective non-catalytic reduction (SNCR) device, or anelectrostatic precipitator device. The ammonia is supplied by applying avoltage difference to an electrolytic cell, which includes a cathodehaving a first conducting component, an anode having a second conductingcomponent, urea, and an alkaline electrolyte composition in electricalcommunication with the anode and the cathode, where the alkalineelectrolyte composition has a hydroxide concentration of at least 0.01 Mor a pH of at least 8. The voltage difference is applied across thecathode and the anode, and the voltage difference is sufficient toeffect the electrolytic hydrolysis of urea to produce at least NH₃.

According to yet another embodiment of the present invention, an exhaustgas treatment system is provided. The exhaust gas treatment systemcomprises at least one of a selective catalytic reduction system, aselective non-catalytic reduction system, or a flue gas conditioningsystem and an ammonia generator. The ammonia generator comprises anelectrolytic cell including a cathode having a first conductingcomponent, an anode having a second conducting component, urea, avoltage source, an alkaline electrolyte composition in electricalcommunication with the anode and the cathode, and an ammonia outlet fromthe ammonia generator in communication with the at least one of theselective catalytic reduction system, the selective non-catalyticreduction system, or the flue gas conditioning system. The alkalineelectrolyte composition has a hydroxide concentration of at least 0.01 Mor a pH of at least 8.

The invention will be further appreciated in light of the followingdetailed description and drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the general description of the invention given above, andthe detailed description given below, serve to describe the invention.

FIG. 1 is a schematic representation of a method to produce ammonia fromurea;

FIG. 2 is a diagrammatical view of a simplified electrolytic cellcoupled to exhausted combustion gases;

FIG. 3 is a diagrammatical view of a method to purify exhaust gases froma combustion engine;

FIG. 4 is a plot of current density at a constant voltage (1.4 V) in theelectrochemical cell over time;

FIG. 5 is a plot of current density at a constant voltage (1.33 V) inthe electrochemical cell over time;

FIG. 6 is a diagrammatic depiction of an electrolytic ammonia generatorsystem according to an embodiment of the present invention; and

FIG. 7 is expanded assembly of an electrolytic flow cell according to anembodiment of the present invention.

DETAILED DESCRIPTION

The SCR or SNCR of nitrogen oxides (NO_(x)) and/or the reduction of flyash is facilitated by the electrolysis-induced hydrolysis of urea and isdescribed herein. Advantageously, the electrolytic cell conditions maybe modified to additionally generate hydrogen, which may be injected toincrease fuel efficiency, to provide heat into the electrolytic cell, orto provide electricity into the electrolytic cell.

Referring now to FIG. 1, urea may be subjected to electrolysis-inducedhydrolysis in an electrolytic device. The electrolytic device maycomprise a cell or multiple cells that each contains an anode and acathode. The electrolytic cell can operate in batch mode, continuousmode, semi-continuous, and with recirculation, as needed to provide ondemand and controlled injection of ammonia into a process gas streamsuch as a combustion gas exhaust. At the anode, the working electrode ofthe cell, urea may be hydrolyzed to ammonia. The overall hydrolysisreaction is provided in Equation 8 below.(NH₂)₂CO+H₂O→NH₃↑+CO₂↑  (Equation 8)

Referring more particularly to FIG. 2, a simplified electrolytic cell 1representing a single batch-type arrangement comprises a tank 2, whichmay be made of light gauge iron, steel, TEFLON®, or other material notattacked by an alkaline electrolyte composition. An electrode assemblycomprising two electrodes, an anode 3 and a cathode 4, is suspendedwithin an alkaline electrolyte composition 6 contained in tank 2.Optionally, a separator 5 may be positioned between the anode andcathode. In this single batch-type arrangement, the alkaline electrolytecomposition 6 includes an effective amount of urea as described below.The anode 3 and cathode 4 are electrically connected to a voltage source7, which provides the electrical energy for the electrolysis of ureacontained in the alkaline electrolyte composition 6. In a batch-typearrangement, the alkaline electrolyte composition may be stirred tofacilitate mass transfer. It will be readily apparent to one of ordinaryskill in the art that the above cell is readily adaptable to acontinuous flow cell configuration, semi-continuous, and withrecirculation of the alkaline electrolyte composition.

The electrodes comprise a conductor or a support which can be coatedwith one or more active conducting components. Exemplary conductorsinclude, but are not limited to, metals such as nickel and platinum,alloys such as carbon steel or stainless steel, or other materialscapable of conducting electricity such as carbon or graphite. Exemplaryelectrode support materials may be chosen from many known supports, suchas foils, meshes, sponges, and beads, for example. The support materialsmay include, but are not limited to, Ni foils, Ti foils, graphite,carbon fibers, carbon paper, glassy carbon, carbon nanofibers, andcarbon nanotubes. Aside from these specific support materials listed,other suitable supports will be recognized by those of ordinary skill inthe art.

Accordingly, the cathode may comprise a conductor that is inert to analkaline electrolyte composition. Additionally, the cathode may furtherinclude a support material that is inert to the alkaline electrolytecompositions and coated with one or more active conducting components.For example, the conducting component of the cathode may include carbon,cobalt, copper, iridium, iron, nickel, palladium, platinum, rhodium,ruthenium, or mixtures or alloys thereof. Exemplary conductingcomponents include carbon steel and stainless steel.

The anode may comprise a conductor that is inert to the alkalineelectrolyte composition. Additionally, the anode may further include asupport material that is inert to the alkaline electrolyte compositionsand coated with one or more active conducting components. According toembodiments of the present invention, the reaction of urea hydrolysisoccurs at the conducting component of the anode. Therefore, theconductor and/or the conducting component at the anode is one or moremetals active toward electrolytic hydrolysis of urea. Active metals mayinclude cobalt, copper, iridium, iron, platinum, nickel, rhodium,ruthenium, or mixtures or alloys thereof, for example, and inparticular, nickel. The active metals may be in an oxidized form, suchas nickel oxyhydroxide.

The structure of the anode is not limited to any specific shape or form.For example, the active metal may be formed as foil, wire, gauze, bead,or coated onto a support.

Exemplary working electrodes include, nickel electrodeposited on acarbon support, such as carbon fibers, carbon paper, glassy carbon,carbon nanofibers, or carbon nanotubes, and nickel formed into beads andsuspended in a nickel gauze.

One electrode found to be favorable to the electrolysis-inducedhydrolysis of urea is an activated nickel oxyhydroxide modified nickelelectrode (NOMN) on different 4 cm²-metallic substrates (Ni foil, Nigauze, Ti foil and Ti gauze) that have been electroplated with 10±0.1 mgof Ni using a Watts bath. Specifically, the plated nickel electrode isactivated by immersed in a solution containing nickel sulfate, sodiumacetate, and sodium hydroxide at 33° C. Stainless steel is used ascounter electrode. The plated nickel electrode may be used as the anodeand cathode by manual polarity switching at 6.25 A/m² for four 1-minutecycles and two 2-minute cycles. Finally, the electrode is kept as theanode at the same current and maintained thereat for two hours. Theactivated electrode yields higher current densities than those of M/Ni,where M represents a metallic substrate.

The separator 5 compartmentalizes the anode and cathode. Separatorsshould be constructed from materials chemically resistant to thealkaline electrolyte composition. Many polymers are suitable forconstructing separators, such as Teflon® and polypropylene. Separatorsare not required for simple batch-type arrangements, but may beadvantageous for continuous flow electrochemical cells or fuel cells.Separators may include ion exchange membranes, solid electrolytes orelectrolytic gels, for example. Separators may be permeable,semi-permeable or impermeable to gases or liquids.

According to the present invention, the electrolyte composition isalkaline and has a hydroxide ion concentration of at least 0.01 M or apH of at least 8. According to one example, the alkaline electrolytecomposition has a hydroxide concentration of at least 0.01 M and a pH ofat least 8. As such, the alkaline electrolyte composition may include asufficient quantity of any suitable hydroxide salt, carbonate salt orbicarbonate salt to provide an electrolyte composition with a hydroxideion concentration of at least 0.01M and/or a pH of at least 8. An alkalimetal hydroxide or alkaline earth metal hydroxide salt, such as lithiumhydroxide, rubidium hydroxide, cesium hydroxide, barium hydroxide,strontium hydroxide, potassium hydroxide, sodium hydroxide, magnesiumhydroxide, calcium hydroxide, and mixtures thereof may be used. Inparticular, the alkaline electrolyte composition includes potassiumhydroxide. Advantageously, the sequestration of CO₂ gas, shown inEquation 1, may be realized by the reaction of CO₂ with hydroxide toform carbonate, which may be retained in the alkaline electrolytecomposition. Similarly, alkali metal carbonates or bicarbonate salts oralkaline earth metal carbonates or bicarbonate salts are also suitableelectrolytes.

The concentration of the hydroxide, carbonate, or bicarbonate salts mayvary according to embodiments of the invention. For example, accordingto one embodiment, the concentration of the hydroxide, carbonate, orbicarbonate salts may be from about 0.01 M to about 8 M. In anotherexample, the concentrations of potassium hydroxide, potassium carbonate,potassium bicarbonate, sodium hydroxide, sodium carbonate, or sodiumbicarbonate from about 2 M to about 8 M and from about 4 M to about 8 M,are particularly effective.

The alkaline electrolyte composition may comprise a gel, such as a solidpolymer electrolyte. Suitable gels include, but are not limited to,those containing polyacrylic acid, polyacrylates, polymethacrylates,polyacrylamides and similar polymers and copolymers.

The electrolytic gel may be prepared using any suitable method. Onemethod includes forming a polymer and then injecting a hydroxide, acarbonate or a bicarbonate salt electrolyte into the polymer to form apolymeric mixture. In another method, the monomer may be polymerized inthe presence of a hydroxide, a carbonate, or bicarbonate saltelectrolyte.

According to one embodiment, the electrodes are separated by theelectrolyte gel which contains an effective hydroxide, carbonate, orbicarbonate ion concentration. The anode is contacted with a ureasolution as the feed stock. The cathode is then contacted with asuitable aqueous solution, such as water or a hydroxide, carbonate, orbicarbonate solution, for example.

Alternatively, the gel electrolyte is not fixed and can flow through anelectrolytic cell. According to another embodiment, urea may becontained within the gel or an aqueous solution comprising urea may flowwithin the gel electrolyte.

In the cell shown in FIG. 2, the electrolyte composition 6 includesurea, which may vary from trace amounts up to about a saturatedsolution, which is approximately 12 M at standard temperature andpressure. Advantageously, the specific source and purity of the urea isnot particularly limited.

Moreover, for the formation of aqueous solutions of urea, the specificsource and purity of the water used in making the aqueous solution isnot particularly limited or critical. One reason for this advantage isthat, according to embodiments of the present invention, the entireaqueous solution comprising urea is not volatilized to thereby leavebehind trace minerals and other non-volatile materials. Instead, themajority of the water remains in the liquid form, which substantiallymaintains the trace minerals in solution. Additionally, after theelectrolytic hydrolysis of at least a portion of the urea within theelectrolytic cell, the aqueous solution or the alkaline electrolytesolution being discharged from the electrochemical cell may berecirculated.

Voltage source 7 may be any available source, such as batteries, fuelcells, power from the grid, and renewable energy sources, such as asolar cell or a wind-turbine generator, for example. When theelectrolytic cell is coupled with an SCR system on a motor vehicle, theelectric source may be from an alternator. In order to attain desiredefficiencies, a voltage sufficient to initiate the electrolytichydrolysis of urea is required. But it is preferable that the voltagenot be so high as to significantly electrolyze water. Generally, theminimum voltage required to electrolyze or electrolytically-hydrolyzeurea is about 0.85 volts. The voltage required to electrolyze water isgreater than 1.7 volts with a platinum electrode at standard conditions,but the rate of electrolysis and/or electrolysis-induced hydrolysisdepends on other factors, such as temperature and ionicstrength/conductivity. Based on the foregoing, the voltage range appliedto the electrolytic cell to electrolytically-hydrolyze urea may be fromabout 0.85 volts to less than about 1.7 volts. The voltage range may befrom about 1.2 volts to about 1.6 volts. Typically, the electrolyticcell will be operated at a constant voltage within these ranges.

Additionally, the rate of producing ammonia and/or hydrogen from ureamay be controlled by varying the voltage within different regions of theelectrolytic cell. For example, in a packed-bed type electrolytic cell,the voltage within the packed-bed of an anodic catalyst material can beadjusted along the catalyst bed to control the rate of ammoniaproduction and/or injection into an SCR or SNCR device. As such,different regions in the catalyst bed may have different potentials tocontrol the rate of ammonia production. For example, a packed bed columnconfiguration may include a plurality of anodes with each beingelectrically insulated from the other anodes and capable of havingvoltage controlled separately thereto, such as that represented in FIG.6. For a given maximum production of ammonia, the totality of the anodesmay be polarized. However, when a lower amount of ammonia is desired,then less than all of the anodes are polarized.

Amperage or current density may affect the performance of anelectrolytic cell, as well. Pure water has poor electrical conductivityand, as such, electrolysis in pure water is very slow and essentiallyoccurs due to the self-ionization of water. Generally, the rate ofelectrolysis increases by adding an electrolyte, such as a salt, an acidor a base. Therefore, the presence of an added hydroxide ion, acarbonate ion or a bicarbonate ion and its respective counter ion, inthe alkaline electrolyte composition enables the conduction ofelectrical current. The current density of the electrolytic celldescribed herein ranges from about 1 mA/cm² to about 500 mA/cm². In someembodiments, the current density range may be from about 50 mA/cm² toabout 400 mA/cm². The current density range may be from about 200 mA/cm²to about 300 mA/cm². Overall, it is only necessary to provide asufficient amount of current to induce the active form of the activemetal, which comprises the anode, to cause the hydrolysis of urea.Typically, the electrolytic cell will be operated at a constant currentor current density within these ranges.

The electrical current may also be used to control the production ofammonia from the electrolytic hydrolysis of urea and therefore controlthe rate of injecting ammonia into an exhausted gas treatment system.For example, a given electrical current may be required to induce theactive form of the active metal in all the regions of the anode tomaximize the production of ammonia. The applied current may be loweredwhen the need for ammonia decreases.

Electrolytic cells may operate over varying ranges of pressure andtemperature. The operating pressure may be about atmospheric pressure orambient pressure with no upper pressure limit other than the physicallimits of the reaction vessel. If desired, the operating pressure of theelectrolytic cell may be varied to control the rate of ammonia that isinjected into an exhaust gas. The operating temperature range may befrom about 0° C. to about 100° C. An acceptable operating temperaturerange may be from about 40° C. to about 80° C. More specifically, anoperating temperature range from about 60° C. to about 70° C. isparticularly useful.

The temperature in the electrolytic cell may be controlled with anyavailable source. For example, the electrolytic cell may further includea heater apparatus operatively coupled to electrolytic cell, and/or arecirculation system operatively coupled to the electrolytic cell,wherein the recirculation system contains at least a portion of thealkaline electrolyte composition. Exemplary heating apparatus includeheating jackets that surround the electrolytic cell, from which heat maybe supplied by external sources, such as steam, heated water, or otherheated fluids. Other possible heating sources can include, but are notlimited to, electric heaters or combustion gases. Alternatively, or inaddition, the recirculation system may also include a heating apparatusfor increasing the temperature of the alkaline electrolyte compositionat a point external to the electrolytic cell. The desired heating sourcemay depend on the availability and/or compatibility with the system. Forexample, electric heat may be the most convenient way to provide theheat to achieve a desired operating temperature for the use of theelectrolytic cell in an automobile SCR system, especially during coldstart and during extreme weather conditions. Accordingly, theelectrolytic cell may have temperature control that is independent ofthe temperature of the engine.

It will be readily apparent to one of ordinary skill in the art that theabove-described electrolytic cell is readily adaptable to a continuousflow cell configuration, semi-continuous, and with recirculation of thealkaline electrolyte composition. For example, an exemplary system forthe continuous generation of sufficient quantities of ammonia toadequately supply the needs of a coal fired power plant on a continuousbasis is shown in FIG. 6. From a urea storage container 10, urea prillis supplied via a rotary feed valve 12 to a mix tank 14 where the ureaprill is mixed with water from a water supply 16 to form a ureasolution. The mix tank 14 includes a discharge line 18, which suppliesthe urea solution to a urea solution feed pump 20 to transfer the ureasolution to a urea electrolyte tank 24 through the urea electrolyte tankinlet 22. A urea solution recirculation line 26 permits continuousoperation of the urea solution feed pump 20. According to thisembodiment, the urea electrolyte composition is formed by mixing theurea solution from the mix tank 14 with an alkaline electrolytecomposition including hydroxide, carbonate, or bicarbonate salts ofalkali metals or alkaline earth metal, or combinations thereof. The ureaelectrolyte tank 24 includes a discharge line 28, which supplies theurea electrolyte solution to a urea electrolyte solution feed pump 30 totransfer the urea electrolyte solution through an electrolytic cellinlet 32 to an electrolytic cell 34. A urea electrolyte solutionrecirculation line 36 permits continuous operation of the ureaelectrolyte solution feed pump 30, and may also participate in controlof the volume or level of urea electrolyte solution within theelectrolytic cell 34. The electrolytic cell 34 includes a heating jacket38 having an inlet line 40 and an outlet line 42 for recirculatingheating fluids therethrough.

One typical flow cell design is that of a packed-bed type ofelectrolytic flow cell, which enables the voltage and/or the currentwithin the packed bed of anodic catalyst material to be varied along thecatalyst bed and thereby control the rate of ammonia evolution. Apacked-bed type flow cell is depicted in FIG. 6 with V1-V6 representingthe variable voltage capability of the electrolytic cell 34, where theinsulating materials between the electrically insulated regions of thepacked anodic catalyst bed are not shown. This configuration is alsoadaptable for controlling the amount of urea being hydrolyzed based onthe level or volume of urea electrolyte solution covering the catalystbed. In other words, varying an area percentage of a total area of theanodic catalyst bed in contact with a urea solution will vary the rateof ammonia production. As such, increasing the amount of ureaelectrolyte solution covering the available catalyst bed will increasethe rate of ammonia production.

During operation, the urea electrolyte solution flows through theelectrolytic cell 34 and thereby contacting the electrodes. Accordingly,the generated ammonia from the electrolytic hydrolysis of urea issupplied to an exhaust gas treatment system through an ammonia dischargeline 44. Depending on the electrolytic cell operating conditions,hydrogen may also be produced and supplied to auxiliary systems througha hydrogen gas discharge line 46. The urea electrolyte solution, afterhaving been depleted of at least a portion of its urea, is returned tothe urea electrolyte tank 24 though urea electrolyte return line 48.

Other flow cell designs are also amenable to the instant embodiment. Asshown in FIG. 7, a flow cell 60 may include a jacketed containmentvessel 62 having a tubular cathode 64, a tubular anode 66 and a vessellid 68. The jacketed containment vessel 62 may be thermally controlledby any suitable method. The jacketed vessel 62 further includes inlet70. When present, a tubular separator 72 compartmentalizes the tubularcathode 64 and the tubular anode 66, which permits separation of theeffluents therefrom. Accordingly, each electrode chamber may have itsown discharge port, whereby the vessel lid 68 is configured toaccommodate a cathode connector tubing 74 and an anode connector tubing76. For example, the cathode connector tubing 74 may be hollow andinclude a conductor to thereby provide both a discharge flow path fromthe proximity of the tubular cathode 64 and an electrical connection.Similarly, the anode connector tubing 76 may be hollow and include aconductor to thereby provide both a discharge flow path from theproximity of the tubular anode 66 and an electrical connection.

The present invention will be further appreciated in view of thefollowing examples.

Example 1

Two closed cells (1000 mL) were assembled. Each cell was filled with 200mL of 7 M KOH and 0.33 M urea solution, and stirred at 120 rpm. Voltage(1.4 V) was applied to cell B (supplied with Arbin Industries MSTAT)using a Rh—Ni anode (0.15 mg/cm² Rh on Ni foil, 10 cm²) and platinumfoil (10 cm²) as the cathode. Samples were taken via liquid samplingports and analyzed for ammonia concentration periodically by extracting10 mL and diluting 1:100 with distilled water. A 50 mL aliquot of thisanalyte was added to a flask, 1 mL of pH adjusting solution was addedwith stirring, and the solution was analyzed using an ion selectiveelectrode. After two hours of constant voltage operation, Cell A and Bcontained aqueous ammonia concentrations of 3600 and 4700 ppm,respectively (Table 1). After 3 hours of operation, cell A increased to3800 ppm while cell B increased to 6450 ppm, which provided that thecell with applied potential had 41% higher conversion of urea toammonia. Cell B averaged about 25 mA/cm² during the first two hours,which decreased to near 8 mA/cm² for the third hour (FIG. 4). Theseresults show that the lower current density was more effective inconverting the urea to ammonia.

TABLE 1 Urea hydrolysis via electrolysis samples. Test Time ppm NH₃ Avg.Current % increase w/ (total hrs) Cell (liquid Phase) (mA) electrolysisA 3637 2 B 4715 98 23 A 3800 3 B 6450 30 41

Application of 1.4 V to cell B resulted in a 41% higher conversion after3 hours of operation, indicating that the urea to ammonia reaction is infact enhanced by electrolysis. Electrolysis at a low voltage contributesto kinetics of the urea to ammonia conversion.

Example 2

Two closed cells (1000 mL) were assembled with Rh—Ni anodes (8 cm² each;cell A: 0.05 mg/cm², cell B: 0.15 mg/cm²) and platinum foil cathodes (15cm²), filled with 7 M KOH and 0.33 M urea, and heated to 70° C. Liquidsampling ports were included for monitoring aqueous ammoniaconcentration ex-situ by ISE throughout the duration of the experiment.Voltage (1.33 V) was applied to both cells A and B (supplied with ArbinIndustries MSTAT) with 120 rpm stirring. A lower voltage was chosen ascompared to Example 1 above because it was postulated that a lowervoltage, which will provide a lower current density, was needed toaffect the NiOOH catalyzed reaction to ammonia.

Samples were taken and analyzed for ammonia concentration periodicallyby extracting 10 mL and diluting 1:100 with distilled water. A 50 mLaliquot of this analyte was added to a flask with stirrer and ISEelectrode and 1 mL of pH adjusting solution, as described inExperiment 1. After two hours of constant voltage operation, Cell A andB contained aqueous ammonia concentrations of 4890 and 6470 ppm,respectively (Table 2). These concentrations did not increase after thethird hour of operation. The average current in each cell for the firsttwo hours was 1.5 and 2.0 mA/cm² for cell A and B, respectively (FIG.5). It is postulated that the apparent stoppage in urea conversion toammonia after the first sample period is likely the result of thecurrent density dropping to around 1 mA/cm² after 2 hours, which may bebelow the level necessary to affect the reaction. It was observed that ablack precipitate formed on the platinum cathode in both cells. Most ofthe conversion affected by applied potential probably took place withinthe first hour where average current was 2-3 mA/cm². Otherwise, leakagefrom the liquid sampling ports could explain the lack of increase inconversion.

TABLE 2 Urea hydrolysis via electrolysis samples. Time Cell A ppm NH₃Cell B ppm NH₃ 2 hrs 4890 6470 3 hrs 4580 6400

Based on these results, the effect of current density on the conversionof urea to ammonia and the effect of catalyst loading Cell B exhibited ahigher conversion than cell A, probably because it had an anode withhigher loading of rhodium and operated under a slightly higher averagecurrent density. Again, these results show that electrolysis at a lowvoltage can contribute to favorable kinetics of the urea to ammoniaconversion.

For example, for a Diesel truck application, providing 0.5 Kg of ammoniaper hour to an SCR unit at a current of 6.25 amps and a cell voltage of1.33 volts, would correspond to 8.31 watts of power. The thermal energyconsumed would be 1,980 kilojoules. Additionally, under theseconditions, approximately 0.23 g/hour of hydrogen may be generated,which equates to about 33 kilojoules of thermal energy, and may beinjected into the combustion engine of the diesel truck to minimizecarbon dioxide emissions and increase fuel efficiency.

In another example, for a 500 MW coal-fired power plant, providing 200Kg of ammonia per hour to an SCR unit at a current of 2,500 amps and acell voltage of 1.33 volts, would correspond to 3.325 kilowatts ofpower. The thermal energy consumed would be 792,000 kilojoules.Additionally, under these conditions, approximately 93.3 g/hour ofhydrogen may be generated, which equates to about 13,228 kilojoules ofthermal energy.

Example 3

Electrolytic Hydrolysis of Urea: A cell containing 7 M KOH/0.33 M ureasolution at atmospheric pressure was subjected to electrolysis-inducedhydrolysis. A cell voltage of 1.4 volts was applied to a 2×2.5 cm²carbon-paper anode deposited with Ni, and a 5×5 cm² Pt foil cathode.Under these conditions, the presence of ammonia was detected from theconversion of urea into ammonia and carbon dioxide. The hydrolysispathway becomes favorable with increasing hydroxide salt concentrationand increasing temperatures. For example, urea samples contained in 0 M,1 M, 5 M and 7 M KOH at 50° C. for 89 hours produced 0.7%, 4.2%, 27.4%and 36.7% hydrolysis, respectively. A 7 M KOH sample of urea at 70° C.for 24 hours provided over 95% hydrolysis.

Example 4

Flow Cell Hydrolysis of Urea: In a sandwich-style urea electrolytic cellthat compartmentalized the anode and cathode, a polypropylene membranewas used as a separator. The anode was constructed of a 5 cm²carbon-paper support, on which was electrodeposited Ni. The cathode wasconstructed of a 5 cm² carbon paper support, on which waselectrodeposited Pt. The electrodes were immersed in 5M KOH/0.33 M ureaat 70° C. A cell voltage of 1.33 volts was applied and ammonia evolvedfrom the anode. It was noted that a small amount of hydrogen wasproduced from the cathode. The respective gases were analyzed using aMG2 SR18610C gas chromatograph with a thermal conductivity detector(TCD), Haysep column, and a molecular sieve column. Pure hydrogen wasobserved at the cathode, while ammonia, N₂ and small amounts H₂ wereobserved from the anode in gas phase. The hydrogen on the anode side ofthe separator is believed to arise from hydrogen passing through thepolypropylene membrane. Ammonia was further detected in the liquid phaseusing an Orion ammonia selective electrode (ISE). No carbon species weredetected in gas phase. It is postulated that any CO₂ that may have beengenerated was quickly transformed into potassium carbonate.

Example 5

Electrolysis of Urea: A cell containing 5 M KOH/0.33 M urea solution at25° C. and atmospheric pressure was subjected to electrolysis. A cellvoltage of 1.4 volts was applied to a 2×2.5 cm² carbon-paper anodedeposited with Ni, and a 5×5 cm² Pt foil cathode. It was determined bygas chromatography that the electrolysis of urea produced nitrogen atthe anode of this electrolytic cell, whereas hydrogen was produced atthe cathode. Ammonia, which is presumably derived from theelectrolysis-induced hydrolysis of urea, was detected in theelectrolyzed solution using an Orion ammonia selective electrode (ISE).No carbon species were detected in the gas phase. It is postulated thatthe generated CO₂ was quickly transformed into potassium carbonate byreacting with potassium hydroxide in the alkaline electrolytecomposition.

Therefore, at the anode, urea may be oxidized to nitrogen and carbondioxide. At the cathode, the counter electrode, hydrogen may beproduced, as shown in the following reaction:(NH₂)₂CO+H₂O→N₂↑+CO₂↑+3H₂↑  (Overall Electrolysis Reaction)

Therefore, in addition to the electrolysis-induced hydrolysis of urea tosupply the requisite ammonia reductant to an exhaust gas treatmentsystem, under the appropriate conditions, the foregoing electrolysis ofurea may provide hydrogen, which may be injected into a combustionchamber that is attached to the exhaust gas treatment system, as shownin FIG. 3. Thus, adding hydrogen to the combustion chamber mayfacilitate improved fuel combustion efficiency, as well as reducingunwanted emission by-products.

Example 6

The model system in accordance with the embodiment represented in FIG. 6has been designed with an electrolytic cell having a total volume of 825liter, with 660 liters of an anodic bed providing 1,247 m² of activemetal surface. Extrapolating the experimental data obtained from abatch-type configuration, operation mass transfer parameters werecalculated for the foregoing system. Additionally, a comparison was madebetween the inventive electrolytic hydrolysis method (EU2A) and thecommonly-used chemical hydrolysis. As shown in Table 4 below, theelectrolytic urea to ammonia (EU2A) hydrolysis method provides anammonia stream which is predominantly (e.g., 64 molar %) comprised ofammonia. The calculated parameters and comparison data are shown inTables 3 and 4, respectively.

TABLE 3 Calculated operating parameters. Stream Rate (kg/hr) Composition(%) 1: Prill urea 352.9 100.0 2: Water 119.4 100.0 3: Concentrated urea472.3 Urea: 74.7 H₂O: 25.3 4: Urea electrolyte reactor feed 1138.0 Urea:31.0 K₂CO₃: 22.5 H₂O: 46.5 5: Electrolyte recycle 665.7 K₂CO₃: 38.4 H₂O:61.6 6: NH₃ to SCR (@ 70° C.; 30 psig) 472.2 NH₃: 42.4 CO₂: 54.7 H₂O:2.9 7: Hydrogen to fuel cell 0.1 100.0 8: Saturated steam (150 psig)394.8 100.0 9: Exhausted steam (150 psig) 394.8 100.0

TABLE 4 Comparison of methods. Description Chemical hydrolysis EU2AVolume of reactor (liters) 7,250 825 SCR Ammonia Reagent 200 kg/hr 200kg/hr Dry Urea Flow Rate 352 352 Reagent Concentration 50% wt 40-60% wtDI water 375 kg/hr 119 kg/hr Steam heating (150 psig) 840 kg/hr 395kg/hr Power N/A 1.8 kW** Gas Molar Composition NH₃ (%) 22.8 64.0 CO₂ (%)11.4 32.0 H₂O (%) 65.8 4.0 **Reusing the hydrogen in a fuel cell with50% efficiency.

One issue commonly encountered in electrolytic cells, is the slowdeactivation of the one or both of the electrodes. In some instances,the deactivation may be attributed to the attachment of an oxidized filmon the anode and/or the attachment of scale on the surface of thecathode. This deactivation process deteriorates the electrolyticefficiency of the cell. For example, as this deactivation occurs, thecurrent density can, in some instances, decrease for a constant appliedvoltage, thereby reducing the rate of electro-oxidation. Alternatively,the current density sometimes can be sustained by increasing the appliedvoltage. In either instance, energy is wasted and the overall efficiencyof the cell is diminished.

From an operational perspective, regeneration of the electrodes byreversing the applied voltage for a period of time can be useful. Thereversed voltage may be the same or different as the operating voltage.The reversal voltage may range from about 0.5 volts to about 2.0 volts.Another suitable reversal voltage may range from about 1.4 volts toabout 1.6 volts.

During regeneration, the period of time for applying a reversed voltagemay vary from just a few minutes to tens of hours. For example, thefirst and second conducting components may both include one or moremetals active toward electrochemical oxidation of urea, therefore eitherelectrode may function as a cathode and produce hydrogen. As such,reversing the voltage is effectively an uninterrupted process, therebyallowing the reversed voltage to be applied for an indefinite period oftime or until deactivation is again encountered. According to theoperating conditions of the electrochemical cell described herein,electrodes may be operated for about 5 hours to about 20 hours beforelosing activity and requiring activation.

Conversely, if the anode's conducting component is comprised of a metalinactive toward electrochemical oxidation of urea, the regeneration maybe achieved in about 1 minute to about 20 minutes at about 1.4 volts. Insome instances, reactivation can be achieved in about 6 minutes at 1.4volts.

For SCR applications, the SCR unit is not particularly limited to anyspecific configuration or catalyst. For example, plate, honeycomb,pellet, bead, fiber or corrugated configurations are suitable for use.Moreover, the catalyst is not limited to any species or form. Forexample, traditional catalysts based on vanadium, titanium, or iron orcopper-promoted zeolite catalysts are suitable for use. Additionally,newer SCR catalysts, such as those disclosed in U.S. Pat. No. 7,527,776by Golden et al. may be used. Similarly, for SNCR applications and/orgas flue conditioning applications, the SNCR unit and/or the particleprecipitator are not particularly limited to any specific design.

Accordingly, the electrolytic cells according to embodiments of thepresent invention may be adapted to couple with commercially availableSCR or SNCR units or flue gas conditioning systems. For example, theelectrolytic cell may be adapted to work with existing ammoniagenerators that thermally hydrolyze urea, or the electrolytic cell maybe designed to be the lone source of ammonia for the exhaust gastreatment systems. Alternatively, the cell and the exhaust gas treatmentsystem, such as an SCR or an SNCR system, may be designed as a combinedunit.

The ammonia may normally be introduced into the exhaust gas prior to anelectrostatic precipitator, an SNCR unit, or prior to contacting acatalyst within an SCR unit. The exhaust gas and the ammonia as areducing agent may be contacted with the catalyst, thereby reducing thenitrogen oxides in the exhaust gas. The optimization of temperatures,pressures, flow rates and the like can readily be achieved by one havingordinary skill in the art of exhaust gas treatment technology.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural reference unless the context clearly dictatesotherwise. As well, the terms “a” (or “an”), “one or more” and “at leastone” can be used interchangeably herein. It is also to be noted that theterms “comprising”, “including”, “characterized by” and “having” can beused interchangeably.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail. Thevarious features of exemplary embodiments described herein may be usedin any combination. Additional advantages and modifications will readilyappear to those skilled in the art. The invention in its broader aspectsis therefore not limited to the specific details, representative productand method and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope of the general inventive concept.

1. A method for supplying NH₃ to an exhaust gas treatment systemcomprising: producing ammonia by an electrolytic hydrolysis of ureaeffected by applying a voltage difference to an electrolytic cellcomprising: a cathode having a first conducting component, an anodehaving a second conducting component, urea, and an alkaline electrolytecomposition in electrical communication with the anode and the cathode,where the alkaline electrolyte composition has a hydroxide concentrationof at least 0.01 M or a pH of at least 8, wherein the voltage differenceis applied across the cathode and the anode, wherein the voltagedifference is sufficient to effect the electrolytic hydrolysis of ureato produce at least NH₃; recovering at least a portion of the NH₃; andtransferring the at least a portion of the NH₃ to the exhaust gastreatment system.
 2. The method of claim 1, wherein the first conductingcomponent comprises carbon, cobalt, copper, iridium, iron, nickel,palladium, platinum, rhodium, ruthenium, or mixtures or alloys thereof.3. The method of claim 1, wherein the second conducting componentcomprises cobalt, copper, iridium, iron, platinum, nickel, rhodium,ruthenium, or mixtures or alloys thereof.
 4. The method of claim 1,wherein the second conducting component comprises an oxidized form ofcobalt, copper, iridium, iron, platinum, nickel, rhodium, ruthenium, ormixtures or alloys thereof.
 5. The method of claim 1, wherein thevoltage difference is within the range from about 0.85 volts to about1.7 volts.
 6. The method of claim 1, wherein the voltage difference isapplied as a constant voltage.
 7. The method of claim 1, wherein theexhaust gas treatment system includes at least one of a selectivecatalytic reduction system, a selective non-catalytic reduction system,or a flue gas conditioning system.
 8. The method of claim 1, furthercomprising: affecting a rate of NH₃ production by varying at least oneof a temperature of the electrolytic cell, a pressure of theelectrolytic cell, an electrical current applied to the electrolyticcell, or a voltage applied to the electrolytic cell; varying a voltageapplied to a portion of the anode, wherein the anode comprises an anodiccatalyst bed; or varying an area percentage of a total area of theanodic catalyst bed contacting a urea solution.
 9. The method of claim1, wherein the alkaline electrolyte composition comprises an alkalimetal or alkaline earth metal salt of a hydroxide, a carbonate, abicarbonate, or combinations thereof.
 10. The method of claim 1, whereinthe electrolytic cell further comprises a heater apparatus operativelycoupled to electrolytic cell, and a recirculation system operativelycoupled to the electrolytic cell, wherein the recirculation systemcontains at least a portion of the alkaline electrolyte composition. 11.A method for reducing nitrogen oxide (NO_(x)) emissions and/orparticulate matter in a combustion exhaust gas during ongoing operation,comprising: injecting ammonia into the combustion exhaust gas upstreamof at least one of a selective catalytic reduction (SCR) device, aselective non-catalytic reduction (SNCR) device, or an electrostaticprecipitator device, wherein said ammonia is supplied by: applying avoltage difference to an electrolytic cell comprised of: a cathodehaving a first conducting component, an anode having a second conductingcomponent, urea, and an alkaline electrolyte composition in electricalcommunication with the anode and the cathode, where the alkalineelectrolyte composition has a hydroxide concentration of at least 0.01 Mor a pH of at least 8, wherein the voltage difference is applied acrossthe cathode and the anode, and wherein the voltage difference issufficient to effect the electrolytic hydrolysis of urea to produce atleast NH₃.
 12. The method of claim 11, wherein the first conductingcomponent comprises carbon, cobalt, copper, iridium, iron, nickel,palladium, platinum, rhodium, ruthenium, or mixtures or alloys thereof.13. The method of claim 11, wherein the second conducting componentcomprises cobalt, copper, iridium, iron, platinum, nickel, rhodium,ruthenium, or mixtures or alloys thereof.
 14. The method of claim 11,wherein the voltage difference is within the range from about 0.85 voltsto about 1.7 volts.
 15. The method of claim 11, wherein the voltagedifference is between about 1.2 volts and about 1.6 volts.
 16. Themethod of claim 11, wherein applying the voltage difference to theelectrolytic cell further comprises varying the voltage difference tocontrol the production of NH₃.
 17. The method of claim 11, wherein theelectrolytic cell further comprises a temperature control systemincluding a heat source.
 18. The method of claim 11 further comprisingincreasing a fuel combustion efficiency of a combustion system, themethod comprising: supplying H₂ to a combustion chamber of thecombustion system, wherein the hydrogen is supplied by: applying thevoltage difference to the electrolytic cell, wherein the voltagedifference applied across the cathode and the anode is sufficient toproduce NH₃ and H₂.
 19. An exhaust gas treatment system comprising: atleast one of a selective catalytic reduction system, a selectivenon-catalytic reduction system, or a flue gas conditioning system; andan ammonia generator comprising an electrolytic cell comprised of: acathode having a first conducting component, an anode having a secondconducting component, urea, a voltage source, an alkaline electrolytecomposition in electrical communication with the anode and the cathode,where the alkaline electrolyte composition has a hydroxide concentrationof at least 0.01 M or a pH of at least 8; and an ammonia outlet from theammonia generator in communication with the at least one of theselective catalytic reduction system, the selective non-catalyticreduction system, or the flue gas conditioning system.
 20. The system ofclaim 19, wherein the first conducting component comprises carbon,cobalt, copper, iridium, iron, nickel, palladium, platinum, rhodium,ruthenium, or mixtures or alloys thereof.
 21. The system of claim 19,wherein the second conducting component comprises cobalt, copper,iridium, iron, platinum, nickel, rhodium, ruthenium, or mixtures oralloys thereof.
 22. The system of claim 19, wherein the voltagedifference is within the range from about 0.85 volts to about 1.7 volts.